1. Field of the Invention
The present invention relates to a circuit topology for attenuator and switch circuits having low loss at radio frequencies.
2. Description of the Related Art
Known attenuator circuits are designed using xe2x80x9cTxe2x80x9d or xe2x80x9cPixe2x80x9d resistive network topologies or configurations. The xe2x80x9cTxe2x80x9d resistive network configuration includes two variable series elements and a variable shunt element connected between the series elements. The xe2x80x9cPixe2x80x9d resistive network configuration includes two variable shunt elements and a variable series element connected between the two shunt elements. In both types of network configurations, a first control signal is connected to the shunt element(s) and a second control signal is connected to the series element(s). While the shunt element(s) control the majority of attenuation in xe2x80x9cTxe2x80x9d type attenuators, the series element(s) control the impedance of the circuit.
For example, FIG. 1 shows a prior art attenuator 100 having a xe2x80x9cTxe2x80x9d resistive network configuration with variable series resistors R1xe2x80x2 and R3xe2x80x2 and a variable shunt resistor R2xe2x80x2. In this device, the minimum attenuation state is achieved when the variable series resistors R1xe2x80x2 and R3xe2x80x2 are at a minimum resistance value and the variable shunt resistor R2xe2x80x2 is at a maximum resistance value. Attenuation is initiated by decreasing the variable shunt resistor R2xe2x80x2 via a control signal CTRL2xe2x80x2 and increasing the variable series resistors R1xe2x80x2 and R3xe2x80x2 via a control signal CTRL1xe2x80x2. Variable series resistances R1xe2x80x2 and R3xe2x80x2 ensure that the attenuator matches the impedance of the circuits connected to the input and the output while variable shunt resistance R2xe2x80x2 ensures proper attenuation.
In digital attenuators, only the full ON and full OFF states of the variable elements are used. In these digital circuits, the variable shunt and series elements typically comprise FETs. The width of the gate for the series FETs is chosen to be wide enough to achieve a low insertion loss at the minimum attenuation level. However, this increased width causes an increase in the parasitic capacitance of the device, which causes an impedance mismatch at relatively high frequencies such as radio frequencies.
An object of the present invention is to provide a circuit for attenuation of radio frequency signals that does not introduce parasitic capacitance that limits the dynamic range and that has a low insertion loss.
According to an embodiment of the present invention, an attenuator includes only variable shunt elements. That is, the attenuator according to the present invention does not include variable series elements. Instead, series transmission lines are connected with the variable shunt elements. The impedances of the variable shunt elements and series transmission lines are designed so that the impedance of the attenuator at the input and output terminals is maintained at a nominal level for all levels of attenuation. According to the present invention, the transmission line is an inductive transmission line that is coupled with the capacitance of the variable shunt elements to produce the desired impedance.
According to a further embodiment of the present invention, each of the variable series elements of a known attenuator topology such as the xe2x80x9cPixe2x80x9d or xe2x80x9cTxe2x80x9d resistive network topologies is replaced by a variable shunt element and a series transmission line. As in the embodiment described above, the impedances of the variable shunt elements and series transmission lines are designed so that the nominal impedance of the attenuator is maintained for all attenuation levels.
The variable shunt elements may comprise Field Effect Transistors (FETs), PIN-diodes, and/or Bipolar Junction Transistors (BJTs). FETs operable at radio frequencies include metal semiconductor FETs (MESFETs), high electron mobility transistors (HEMTs), and pseudo-morphic HEMTs (pHEMTs). BJTs operable at radio frequencies include Heterojunction Bipolar Transistors.
The inventive attenuator circuit may be used in digital attenuation circuits, variable attenuator circuits and switches.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.